If you are not going to be the only person using the ski then you need to stay away from the Strada bindings. Like @ToddL mentioned above, those bindings come in sizes like 8, 9, 10, etc.
The Vector boots come in size Small, Standard and Large. The Standard is going to fit a shoe size range from 7-11. That's going to be the best fit if your sisters are going to also use the ski.
You also need to pay very close attention to the plate that the Vector is mounted on. There are two options from Radar; and aluminum plate and what they call the Feather Frame. The rear Feather Frame from Radar will only fit a 2011 or newer Radar ski. I understand that you want to buy a Senate or a Strada, but if you get either of those in a 2010 model and you buy a rear Feather Frame Vector boot, none of the hole will line up.
Having said all that, a pair of Vectors on a Senate is a great setup and very well rounded if more than one skier will be using it.